1. Field of Invention
The present invention relates to a satellite signal reception device that receives satellite signals from positioning information satellites such as GPS satellites, and to a control method for a satellite signal reception device.
2. Description of Related Art
GPS satellites that circle the Earth on known orbits are used in the Global Positioning System (GPS), which is a system for determining one's location, and positioning devices that determine the current location of a receiver that receives signals from GPS satellites are now commonly used and available.
Each GPS satellite has an on-board atomic clock, and each GPS satellite can therefore maintain extremely accurate time information (referred to herein as the GPS time or satellite time information).
GPS receivers that acquire and display positioning information and time information by receiving signals (navigation messages) from GPS satellites have therefore been proposed. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2003-279637.
When the number of GPS satellites from which signals can be received (referred to herein as “receivable satellites” or “captured satellites”) is less than the number of satellites required for positioning (that is, 3 or more), however, the GPS receiver taught in JP-A-2003-279637 continues the satellite search until a preset time-out period (such as 2 to 3 minutes) has ended.
While the time-out time from the start to the end of reception is therefore set to approximately 2 to 3 minutes, particularly in the cold start mode in which reception begins when the GPS satellite orbit information (almanac data) is not known to the GPS receiver, battery power is consumed needlessly if reception continues when the number of receivable satellites is less than the number required for positioning.
For example, if the positioning process is executed when there is only one receivable GPS satellite, searching will continue for the 2 to 3 minute duration of the time-out period because the number of satellites required for positioning cannot be found (captured). Because power consumption by a GPS receiver is particularly high during the satellite search process, battery power continues being consumed as the satellite search and power consumption continue for the 2 to 3 minutes of the time-out period, the duration time therefore becomes shorter, and the system may shut down.